hosting slice 1b, lane A: one-command join, the full lab verdict, the OS sandbox #1762

Part of #1761 (lane A: "a joinable host that does exactly the lab's work"), itself part of #1756. Lane B (economics) runs in parallel.

The decisions are the three owner-approved Lane A rows of #1761; the rules are gHashTag/t27 specs/hosting/{toolchain,host,proof,placement,row}.t27 (gHashTag/t27#8753), vendored byte-identical.

Deliverables

1. One-command join. The prerelease trios-host-v0.1.0 in this repository carries trios-host for darwin-arm64, linux-x64 and linux-arm64, plus t27c and t27b built from one pinned t27 commit, and SHA256SUMS. trios-host join --queen <url> downloads t27c, t27b (arm64 only) and zig, checks each against toolchain.t27, and refuses a corrupted file.
2. The full lab verdict. A shard row is two jobs: the reference half (t27c test-report) and the t27b half (t27b corpus). The t27b half is placed on arm64 hosts only. A row is complete when both halves are agreed, and it is served in the lab's run.json row shape. Acceptance: for at least 20 specs at one commit the lab ran, rows from two local hosts equal the lab's rows, and every difference is named.
3. The OS sandbox. The host measures its isolation level with a planted job, which tries to open a socket and to write to $HOME, and reports the level at registration. A public job needs no network and job-dir-only writes. macOS uses sandbox-exec with a deny-default profile; Linux uses bwrap, else unshare. The planted job fails inside the sandbox and succeeds outside it, which is the negative control.

Base: hosting-mvp. Never queen, no deploy, no Railway change.

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